1. Field of the Invention
This invention relates to a control apparatus for a rotary body, such as a cooling fan or the like, for cooling an engine.
2. Description of Related Art
Generally, to speed up cooling action of a radiator, an engine or electrically driven fan or fans are used to draw air through the radiator core and blow it to an automobile engine. Since an engine driven fan can easily cause a great deal of air to move, it is widely used for large displacement automobile engines.
For such an engine driven fan, it is essential to reduce the loss of engine output efficiency, which accompanies driving and stopping the fan according to various cooling demands for the engine. The loss of engine output efficiency should be as small as possible. Additionally, in order for the engine driven fan to operate with the most cooling efficiency, it is necessary to control the speed of rotation of the fan according not to the speed of rotation of the engine but to certain operating conditions. Such operating conditions may include cooling demands of the engine when no change in cooling demand of the engine is needed although the speed of the engine has changed, a need for a change in cooling demand of the engine although the engine does not change its speed, and a change in the cooling demand for the engine and a concurrent change in the speed of the engine without any correlation therebetween.
A fan speed control system meeting the above-mentioned requirements has been proposed. In such a system, fan speed is controlled in accordance with engine operating conditions, for instance cooling demands of the engine. The fan speed control system has a speed variable means, such as a planetary gear means, through which an engine driven fan is operationally connected to an engine crankshaft so as to enable the engine driven fan to vary in speed and an electromagnetic clutch installed in a carrier element of pinion gears of the planetary gear means which functions to control the speed ratio between the engine crankshaft and the fan. The electromagnetic clutch is, on one hand, locked and unlocked so as to drive and stop the fan and, on the other hand, changes slippage even in a locked condition according to control current supplied thereto. Such a fan speed control system is known from, for instance, Japanese Unexamined Utility Model Publication No. 55-108,218.
Although a fan speed control system, in which fan speed control is performed by a combination of a planetary gear means and an electromagnetic clutch, can perform starting and stopping of the fan with certainty it is not considered practical to control, and particularly linearly control, the speed ratio between the engine crankshaft and the fan, by controlling current supplied to the electromagnetic clutch. This is because linearly controlling the speed ratio between the engine crankshaft and the fan requires advanced techniques due to the complexity in structure and function of the electromagnetic clutch.
In addition, because rotation resistance is simply added to the carrier element of the pinion gear of the planetary gear means by the electromagnetic clutch, and since the speed ratio between the engine crankshaft and the fan, provided by the planetary gear means, is a minimum when the carrier element stops, the planetary gear means can not provide any speed ratio less than the minimum speed ratio due to its structure. Therefore, because a maximum rotation speed of the fan is definitely determined by a speed of the engine and a gear ratio of the planetary gear means, it is difficult to certainly obtain higher speeds of the fan at low engine speeds, in other words, to certainly provide the most suitable speeds of the fan in response to changes in cooling demand of the engine. This difficulty causes the fan speed control system using a planetary gear means to remain unsatisfactory in terms of improvement in cooling performance of the engine.
In the fan speed control system, if the planetary gear means for variably controlling a speed ratio malfunctions, the fan will not reach a speed sufficient to cool the engine depending upon a cooling demand of the engine. This causes overheating of the engine in some cases. For this reason, there is a requirement that the fan speed control system be equipped with a measure to secure desired fan speeds even during such a malfunction of the variable speed ratio control means, such as the planetary gear means, of the fan. However, there has not yet been found any effective technique to satisfy this requirement.
There is also a requirement for the variable speed ratio control means, such as the planetary gear means, of the fan to be properly lubricated for durability and reliability owing to its working efficiency or operation time, which is very high or long. However, no effective lubricating means has so far been proposed.